Liquid ion-exchanger composition containing boron oxide and an alkali metal oxide



United States Patent 3,538,010 LIQUID ION-EXCHANGER COMPOSITION CON-TAINING BORON OXIDE AND AN ALKALI METAL OXIDE Monte H. Rowell, 1520Everett St., El Cerrito, Calif. 94530 No Drawing. Original applicationJuly 27, 1964, Ser. No. 385,519, now Patent No. 3,337,306, dated Aug.22,

1967. Divided and this application July 17, 1967, Ser.

Int. Cl. B01j 1 04; C09k 3/00 US. Cl. 252182 7 Claims ABSTRACT OF THEDISCLOSURE Liquid ion-exchanger composition adapted for use attemperatures ranging from 750 C. to 1400 C. Ion exchange between twoliquid phases is accomplished by placing the ion-exchanger compositionin contact with a fused salt with which it is immiscible, separating thetwo phases when the desired exchange of cations has taken place, andrecovering the exchanged ions from the fused glass phase.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

The present invention is a division of patent application Ser. N0.385,519, filed July 27 1964, and now Pat. No. 3,337,306 issued Aug. 22,1967 and relates to a hightemperature liquid-liquid extraction processand particularly to such a liquid-liquid extraction process which ispeculiarly adapted for use with ionic melts such as molten salts(sometimes referred to as fused salts) or any other liquids which arestable at the high temperature levels which characterize molten or fusedsalts.

Liquid-liquid extraction, per se, is established in the art and ingeneral terms involves two liquid phases which are in contact with eachother and which are substantially immiscible with respect to each other.Based upon such diverse mechanisms as (a) preferential solubility or (b)ion exchange across the interface between the two liquid phases, varioussolutes will assume a preferential distribution within the one liquidphase or the other, which, in turn, is the key to the particularextraction sought. The overall obective of the extraction process may beto separate various solutes from one another, to remove various solutesfrom the material(s) representing one of the two immiscible liquidphases, to purify the material representing one of such liquid phases,or some other like purpose attainable with the given liquid-liquidextraction process.

Prior liquid-liquid extraction processes generally have involvedliquid-liquid extraction between aqueous systems arid organic solvents.These prevailing liquid-liquid extraction processes can be regarded asgenerally water or aqueous systems, that is, one of the immiscibleconjugate liquid phases is or contains water. In contradistinction toaqueous systems, a non-aqueous solvent system which recently hasattracted widespread interest is one which involves other liquidsreferred to as molten salts or fused salts. A fused or molten salt,sometimes referred to as an ionic melt, may be thought of as a fluid ofions. Although the fused salt is a liquid which may appear like water,chemistry in fused salts is substantially different from chemistry inwater and is characterized by many advantages not present in an aqueoussystem. For instance, there is a much greater concentration of ions in afused salt than in an aqueous system. Also, fused or molten saltsinvolve much higher temperaice tures and, accordingly, are much morehighly chemically reactive. (It is a rule of thumb that for every 10rise in temperature there is a doubling of chemical reaction rate.)Still another significant advantage of fused salts over aqueous systemsis that there are many instances when things will dissolve readily infused salts though they are very insoluble in water. Therefore thesematerials are much more easily put into solution in a fused salt systemthan in an aqueous system.

The liquid-liquid extraction process of the present invention appears torest primarily upon the mechanism of ion exchange of particularsignificance it incorporates a liquid ion-exchanger which is stable athigh enough temperatures (up to 1400 C.) to be generally employable withfused salts. There has been a previously-developed solvent extraction offused salts which involved tributyl phosphate and a mixture of lowmelting alkali nitrates, but the liquid-liquid extraction processinvolved can be employed only with low-temperature fused salts which donot exceed aobut 300 C. in temperature. In contrast, the liquid-liquidextraction process of the present invention can be employed with fusedsalts to a temperature of about 1400 C. Another very significantdistinguishing feature of the liquid-liquid extraction process of thepresent invention is that the one liquid phase thereof, which coactswith the fused salt constituting the other liquid phase of theextraction process, is immiscible with many fused salts. Sinceimmiscibility of the two conjugate liquid phases is a cardinalrequirement of any given liquid-liquid extraction process, thisbroad-range immiscibility with respect to fused salts endows the presentextraction process with a very wide range of applicability.

Turning now, with particularity, to a specific embodiment of the presentinvention, the liquid-liquid extraction takes place between an ionicmelt, such as a fused salt, as one liquid phase and a fused borate glassas the other phase of the extraction process. The fused salt phase canconsist of any of the wide variety of fused salts with which the fusedborate glass phase is immiscible. Preferably, the fused borate glassphase acts as a liquid ion-exchanger and is formulated in the embodimentby a combination of (1) an alkali oxide generically designated by theformula M 0 where O is oxygen and M is any of the alkali metals such assodium (Na), potassium (K), etc. and (2) boron oxide (B 0 This M 0system forming the preferred borate glass liquid phase of the presentextraction process can take any combination of the above-citedconstituents within the limits set forth in the following Table Iwherein the amounts are specified in mole percent:

TABLE I M Oa fraction of 1%-40% B O -60%almost 100% Within these limitsspecified above in Table I, the most useful composition range forseparations based upon the ion exchange mechanism of selectivity is asspecified in the following Table II which also is in terms of molepercent:

TABLE II Optimum results are exhibited where sodium (Na) is the metal(M) employed in the formulation. Sodium (Na) and potassium (K) willgenerally be preferred because of their greater availability. However,it is not necessary that the cations of the salt and exchanger be alkaliions. Other possible cations include Cu' Cu Ag+, Zn++, Cd, Hg++, Hg+,Tl+, Sn Al+++, and

Pb++. Alkaline earths may be employed at higher temperatures, i.e.temperatures high enough to melt their borates. The essential conditionsare that (a) the borates of the cations should form two immiscibleliquid phases at the same temperatures, and (b) they should bereplaceable from the borate network.

The maximum affinity of the fused borate glass phase (ion-exchanger) formost of the highly charged ions occurs within the formulation ranges setforth in Table III below. Maximum separation factors betweenchemicallysirnilar items also would usually occur within these Table IIIranges.

TABLE III M (alkali oxide)--30 mole percent B O 70-75 mole percent Inthis borate glass liquid phase is formed a boronoxygen network (to bedescribed in detail, infra). The constitutent formulation orsubstitution of this phase, as described above in Tables I, II, and IIIcan be modified by the addition of other elements which form glassypolymeric networks with oxygen such as silicon (Si) and phosphorous (P).Such elements may also be employed as partial substitutes for boron (B)in the given phase formulation. Such su-bstitutional replacement ofboron (B) in this borate glass phase can only be partial and notcomplete, for the presence of boron (B) in this phase is critical to theextraction process of the invention. As far as the effectiveness of theextraction is concerned, it is not necessary to replace any of the boron(B) in this borate glass phase with any of these other possible partialsubstitutes. Boron (B) presently offers the optimum properties relativeto the general extraction effectiveness. However, the empolyment ofsilicon (Si) or phosphorous (P) may be desirable with a view towardshifting certain of the physical characteristics (melting point,solubility etc.) of this borate glass phase. For instance, the additionof silicon (Si) will generally raise the melting point of the phase anddecrease its solubility. Both of the constituents of the M O-B O systemwhich comprise the borate glass phase are in the fused state. There isnothing critical about the formulation procedure. M 0 and B 0 may bemelted prior to being added together or thereafter.

This fused borate glass liquid phase can be employed with any moltensalt or mixture of molten salts which is compatible with the glass phaseat the temperature used, that is, that, at the temperature involved, theborate glass liquid phase and the fused salt liquid phase are immisciblewith respect to each other and are both stable. There is no criticalitywith respect to the relative amounts of the fused borate glass phase andthe fused salt phase except that there must be a sufficient amount ofeach phase present to exceed that amount which is necessarily lost tothe other phase through mutual solubility.

So far the discussion herein has been directed mainly to liquid-liquidextraction in general terms and to the make-up of the two conjugateliquid phases which comprise the high temperature liquid-liquidextraction process defined herein. This discussion will now be directedto an explanation of the operation of the extraction process of theinvention.

The extraction process appears predicated principally upon ion exchangebetween the two liquid phases described above (i.e., between the fusedborate glass phase and the fused salt phase), ion exchange, as is known,being based on a greater chemical afiinity in the one phase for ionspresent in the other phase. The result is that such attracted ions willmigrate across the interface of the two phases and assume a preferentialdistribution in the attracting phase.. In general terms, the attractingor extracting phase is the fused borate or exchanger phase while thephase which relinquishes ions or solutes, as they are more genericallytermed, is the co-phase or fused salt phase. As attracted ions migrateacross the interface and assume their preferential distribution in theextracting (fused molal concentration (of the solute) in the fused saltphase The extraction process of the invention is founded upon this factof preferential distribution of given solutes in the fused borate liquidphase (based primarily on the mech anism of ion exchange) coupled withthe additional circumstance that chemically similar elements such as,for example, Ba, Ca and Sr have relatively large differences in theirdistribution coefficients (K There also is variation in the distributioncoefiicient (K of each solute with variation in the composition of thefused borate phase, thereby adding still greater versatility to theextraction process. Large changes in the distribution coeflicients ofthe solutes are possible by changing the oxide composition of the M O-BO system.

B 0 is believed to form a polymeric network of B and O atoms. If theso-called fused borate phase of the extraction process of the inventionconsisted solely of pure molten B 0 there would be a distribution ofsolutes in the two conjugate liquid phases which would be based onlyupon preferential solubility. B 0 by itself, will not induce an ionexchange betweenthe fused borate phase and the fused salt phase, butwill show preferential solubility toward lesser charged and unchargedspecies. The addition to the B 0 polymeric network of a modifierconsisting of an oxide of one of the alkali metals (preferably either NaO or K 0) converts the B 0 network to a highly-effective ion exchanger.For most solutes, the ion-exchanger reaches its maximum effect at 25-30mole percent alkali oxide. When this alkali oxide is added to thesystem, the oxygen of the alkali oxide enters the boron-oxygen networklargely by covalent bonding, leaving each alkali ion relatively mobile.Negatively charged sites are produced in the borate network by theinclusion of added oxygen. Definied below are a pair of functionalgroups of this borate network with the negatively charged site in eachfunctional group being generally indicated by a negative sign encircledin dotted lines, such One of the mobile alkaliions, each of which has apositive charge, will assume a position in the vicinity of each of thesenegatively charged sites in the borate network to neutralize thenegative charge of each of the charged sites. Below is a representationof each of the charged sites (noted above) in conjuction with itsaccompanying alkali ion M+, designated with a positive charge shownthusly This molten borate glass network has been found to contain onlythe two functional groups noted above and is believed to be athree-dimensional network comprising interlinked functional groups asshown with their associated alkali cations. The showing below isrepresentative of the comprehensive network:

The alkali-modified borate glass network constitutes the fused borateglass liquid phase which coacts with its conjugate ionic melt or fusedsalt liquid phase to accomplish the liquid-liquid extraction of theinvention. The alkali-modified borate glass network acts principally asan in-exchanger-exchanging the alkali ion M+ for other cations which areoriginally present in the ionic melt or fused salt liquid phase. Thealkali-modified borate glass network in effect acts as a captor networcapturing cations from the ionic melt or fused salt phase and giving upalkali metal cations M+ in exchange therefor. The relinquishment of thealkali metal M+ cations in exchange for the captured cations from theionic melt or fused salt phase is a result of obedience of the ionexchange process to the law of electrical neutrality.

The captor network of the fused borate glass phase does not exchange itsalkali cations for all cations in the ionic melt or fused salt phasewith the same degree of preference. This captor network captures givencations from the ionic melt or fused salt phase according to the fieldstrength and other properties of the given cation involved. Fieldstrength of the given cation is directly proportional to its charge andinversely proportional to the outer radius of the cation. In accordancewith this preferential chemical afiinity for cations which is predictedlargely upon cation field strength, the captor network of the motlenborate glass phase will attract a cation A, which has a like charge tocation B but with less ion radius than cation B, before it will attractcation B. The alkali ion M+, for which the captor network has littlechemical affinity when there are competitive ions available, will bereadily replaced at its exchange site" (the network site where anetwork-captured ion replaces an alkali ion M+) in the borate glassnetwork by the network-preferred cation which has migrated across thephase interface from the ionic melt or fused salt phase into the fusedborate glass phase. Stated in terms of distribution concentration, withfused borate glass as the one liquid phase of the extraction process andthe ionic melt or fused salt as the other liquid phase thereof, thedistribution coeflicient K, is low for the alkali metals. The cationswhich migrate from the fused salt or ionic melt phase to be trapped atthe exchange sites of the fused borate network are generally held by thecaptor network by ionic bonding (i.e., by coulombic forces), although arelatively fewer number may be held by covalent bonding.

The mechanism of ion exchange, while extremely important, is not theonly factor influencing the distribution of solutes between phases. Inaddition to cations attracted to functional groups in the borate phasethere will be other cations along with their corresponding anionsdissolved in the borate phase, due to the considerable solubility ofliquid salt in the liquid borate. The solubility of salt in the boratephase increases with increasing alkali oxide content. This solubility isanalogous to the well-known phenomenon with organic resin ion exchangersknown as electrolyte invasion. At low-alkali compositions the boratephase exhibits preferential solubility toward uncharged species andfinds ionic species more repulsive the higher the field strength.Addition of alkali oxide reverses the latter tendency and diminishes theformer tendency. Thus for each solute species there is a characteristicmanner in which K value changes with alkali oxide content. The extrememagnitude of these K value changes and their variability according tosolute are the basis for separations.

Chemically similar elements such as, for example, Ba, Ca, Sr have beenfound to have relatively large differences in their distributioncoefficients (K Having such a marked preferential attraction separationis made a practical reality.

The ability to further vary the given distribution coeflicients of theparticular solutes by varying the composition of the fused borate phaseadds still another dimen Sion to the versatility of the process and thecapability of separating elements from one another, even though they mayhave close chemical similarity. To illustrate the degree of controlledpreferential attraction attainable by the ion-exchanging fused boratephase, a separation factor of 2:1 is attainable by the invention betweenthe respective rate earth elecents europium and neodymium which arecharacteristically considered extremely difficult of separation from oneanother. The respective distribution ratios (between the ion-exchangerphase and the fused salt phase) are 200021 for neodymium and 400021 foreuropium.

The Table II and III formulation ranges have been discussed above interms of the ion-exchange mechanism upon which they are principallypredicated. Other compositions falling within the broader limits definedby Table I may be useful for separation of solutes whose distributiondepends largely or partly on other mechanisms such as preferentialsolubility. For example, neutral species (nonionic) or anionic species(negatively charged) may be separated from cations in most anycomposition region. The loW-alkali region is useful in practicalapplication in making back-extraction (removing solutes from the boratephase). For example, at 20% Na O content, rare earths such as Nd or Euhave K s of about 100. By doubling the B 0 content the Na O contentbecomes 10% and the K is reduced to about 0.1, allowing removal of thision to the salt phase.

( molal concentration of solute in borate phase d molal concentration ofsolute in ball phase When such glass-formers as Si or P are added to thefused borate glass phase or are partially substituted for the boronalthough their precise mechanism-of-action is not known, it isconsidered that they probably participate in the same manner as boron inthat they can be a part of the various functional groups. These otherglassformers such as Si or P may be incorporated into or sub stitutedfor the boron-oxygen network as long as only the two stable liquidphases of the invention process result.

With the highly-controllable preferential distribution ratios andseparation factors available with the extraction process of theinvention and the ability of the extraction process to be employed attemperatures up to 1400 C. and with a wide range of fused salts withwhich the ion-exchanger or fused borate phase is immiscible, it readlycan be seen that the extraction process of the invention represents avast improvement in the field of liquid-liquid extraction.

In the formulations set out supra, M 0, as well as being any alkalioxide, can also be any combination of alkali oxides.

It is also to be pointed out, by way of variation possibilities, thatborate glasses can function as solid ionexchangers, as well as in theform of liquid ion-exchangers. The borate glass phase herein isgenerally preferable in the liquid state as defined above, for theformation of a solid borate glass phase in the borate ionexchanger mayslow its rate of exchange.

What is claimed is:

1. A liquid ion-exchanger, which is stable at temperatures up to 1400 C.and which is employable as the conjugate liquid phase with a fused saltphase in a liquidliquid extraction process, consisting essentially of amolten formulation of from a fraction of a molar percent to about 40molar percent of an alkali metal oxide and from about 60 molar percentto almost 100 molar percent of boron oxide.

2. A liquid ion-exchanger, which is stable at temperatures up to 1400 C.and which is employable as the conjugate liquid phase with a fused saltphase in a liquidliquid extraction process, consisting essentially of amolten formulation of from about 15 to about 33 molar percent of analkali metal oxide and from about 67 to about 85 molar percent of boronoxide.

3, The liquid ion-exchanger of claim 1 wherein said alkali metal oxideis sodium oxide.

4. The liquid ion-exchanger of claim 2 wherein said alkali metal oxideis sodium oxide.

5. The liquid ion-exchangerof claim 2 wherein said ion-exchanger furthercomprises a selected amountof silica, the amount of silica selectedbeing that amount which will adjust the melting point or adjust themutual solubility of said ion-exchanger and said salt phase to the pointdesired. r I

6. The liquid ion-exchanger of claim 2 wherein said ion-exchangerfurther consists essentially of a selected amount of phosphor, theamount of phosphor anhydride selected being that amount which willadjust'the melting point or adjust the mutual solubility of saidion-exchanger and said salt phase to the,point desired. I

7. A liquid ion-exchanger, which is stable at temperatures up to 1400 C.and which is employable as the conjugate liquid phase with a' fused saltphase in a liquidliquid extraction process, consists essentially of amolten formulation of from about 25 to about molar percent of an alkalimetal oxide and from about to about molar percent of boron oxide.

LEON D. ROSDOL, Primary Examiner I. GLUCK, Assistant Examiner

